Extreme ultraviolet lithography (EUVL) involves using EUV radiation (typically 13.5 nm+/−2%) generated by an EUV radiation source to irradiate a reflective patterned mask to transfer the pattern onto a photoresist layer supported by a silicon wafer. The use of the small wavelengths associated with EUV radiation allows for the minimum feature size of the imaged pattern to also be small, i.e., as small as 15 nm and below.
There has been a significant effort to create a source collector module (SoCoMo) based on a laser-produced plasma (LPP) with a multilayer coated collector (MCC), which reflects 13.5-nm light at nearly normal incidence angles and so is sometimes called a normal-incident collector (NIC) mirror. The LPP-MCC SoCoMo has proven to be a highly problematic source-collector solution on the path to developing a commercially viable extreme ultraviolet (EUV) lithography system (tool), which is required to deliver high power (i.e., hundreds of watts of EUV radiation at 13.5 nm+/−2%) to an intermediate focus (IF).
The conventional LPP-MCC approach to creating an EUVL SoCoMo suffers from multiple problems: First, there is the vulnerability of the MCC. The LPP plasma environment can severely limit the lifetime and reliability of the MCC. Second, the power delivered from the LPP source to the IF aperture is inadequate. Third, the laser infrared (IR) radiation reflected from the LPP source can be collected by the MCC and delivered to the IF aperture, causing severe problems to downstream optics and lithography components.
To date, the LPP-MCC SoCoMo has proven unreliable because of the vulnerability of the MCC to the high-power EUV source environment. In particular, the multilayer coatings have proven problematic because they quickly degrade because of mixing of the multilayer coating caused by fast ions from the LPP source, and because their performance is also adversely affected by the deposition of tin (Sn) from tin vapor from the LPP source. A very thin Sn layer on the MCC (e.g., 5 nm thick) can drastically reduce the MCC's performance, and such a thin Sn layer can form in less than 1 minute during system operation.
In this regard, attempts to increase the LPP source power increases the risk to the MCC. In addition to the vulnerability of the MCC, another problem of the LPP-driven EUV source is the production of significant amounts of reflected laser light, which can be collected and directed to the intermediate focus, causing problems at the illuminator, the reticle and the wafer. In this regard, attempts to increase the LPP source power also increase the production of reflected IR and associated problems.
These problems have led to an LPP operational situation in which protecting the MCC has become the highest priority. As a result, it has become highly problematic to optimize the LPP-MCC SoCoMo to achieve the required high-power performance and reduce the collection of reflected laser light under commercially viable operating conditions for EUV lithography.
It would be advantageous to eliminate the vulnerability of the MCC as a design constraint in the SoCoMo and thereby increase reliability and allow for the optimization of the LPP source (including reduced collection of reflected laser light) for the high-throughput operation needed for a commercially viable EUV lithography system.